The invention relates to a method and a device for measuring dust and aerosols in streaming gases using the triboelectrical principle, with its signaling effect being amplified by taking appropriate measures as regards process-engineering and hardware design.
To use the invention, a partial stream is branched off from the main stream and conditioned in a way deemed fit for the purpose, after which the dust and aerosol content is determined by making use of the triboelectrical principle.
In their technical paper xe2x80x9cGrundlage Ladungsdifferenz, triboelektrische Staubmessung unter theoretischen and praktischen Gesichtspunktenxe2x80x9d 1, published as reprint from xe2x80x9cVerfahrenstechnikxe2x80x9d 12/97, H Fodisch and D Richter suggestxe2x80x94among other thingsxe2x80x94that charge transfer occurs when two bodies are brought in contact to each other by touching or rubbing. The surface atoms exchange their electrons so that a boundary layer is created with a positive and a negative surface charge being very close to each other, in a molecular scale. The differential charge forms the basis of triboelectric measuring devices. Among other things, the impact of the dust particles"" velocity on the measuring result is discussed in the above paper, and the findings suggest that after calibration the dust concentration can be determined by combining triboelectric signal and velocity for a certain measurement problem.
A novel dust measuring device, based on the triboelectrical principle, was introduced by H Fodisch, P Schengber and U Riemann, as published in xe2x80x9cVDI Berichtexe2x80x9d, N 1443, 1999, p. 447-458. To this end, reference is also made to the publication of the unexamined application DE 197 29 144 A1, according to which highly precise measuring results are obtained due to the selected probe design in connection with an appropriate scoring hardware. The related dust concentration measuring device is employed for continuous dust content measurements, in particular for pollution control and determination of dust concentration in process monitoring applications. Also, it should be underlined that above all the impact of the dust particles"" velocity on the triboelectric signal was evaluated in an appropriate way so that reproducible results could be obtained.
Furthermore, reference is made to the printed patent specification DD 297 902 A7. The invention is based on the problem of automatically sampling a partial stream from a solid matter laden gas main stream, allowing identical dispersion conditions in both the gas main and partial stream in view of the measuring process to follow.
The reason why reference is made to the printed patent specification DD 297 902 A7 is because also in this invention, to be described in greater detail herein below, a gas stream is branched off and measurements taken after the stream is treated in an appropriate way. To all intents and purposes, sampling a partial stream is an extractive dust measuring method.
Taking into account the prior art, this is to suggest a method and a device for measuring dust and aerosols by which, after sampling a partial stream, the triboelectric effect is used with the object of obtaining a higher signal quality and an improved method for scoring the measurements.
According to the invention the problem is solved as described herein below.
The solution to the problem is now described in further detail. As already mentioned, rubbing two bodies against each other will create a positive and a negative charge, with the difference between these two charges being the fundamental effect triboelectric measuring devices are relying on. The level of the potential difference, that is to say the amount of charge exchanged, depends on a great number of factors. One dependence results from the materials"" electric properties. The much more decisive factors, however, are of a mechanical nature, such as the streaming particles"" velocity, the duration of contact, the size of exchange area, the rubbing intensity and the frequency of contacts. Therefore, the inventive proposal, forming the basis of a new generation of triboelectric measuring devices, works on the assumption that it should be possible to adjust stream conditions and flue gas boundary parameters is such a way as is suited for determining dust content, aerosols concentration and/or dust and aerosols together in the stream branched off. This will be achieved by specifically varying the partial gas stream""s temperature in such a way that aerosols alternatively evaporate or exist in form of liquid drops that can be measured by using the triboelectric effect. As regards the technical implementation, the partial gas stream is heated andxe2x80x94if requiredxe2x80x94diluted to a definite mixture by adding dry, pre-heated, dust-free air.
In this invention, the intensity with which dust and aerosol particles are rubbing across the sensors, that may come in different shape and surface types, is increased by some kind of particle cycloning when they are moving through the triboelectrical measuring chamber.
In contrast to in-situ measurements, in which the gas main stream may have diameters of several meters which in turn makes it difficult to control its flow rate, it now becomes possible to maintain constant conditions as regards the partial stream branched-off and considerably increase its velocity when entering the measuring chamber. Due to the stream""s cyclonic motion, a large distance is covered inside the measuring chamber. Hence, both duration and intensity of rubbing as well as frequency of contacts can be directly controlled and changed if needed.
The triboelectrical measuring chamber may be arranged immediately inside as well as outside the gas main stream. In all cases, the signal recording will be effected by one or more measuring probes, all of them being subjected to the same flowing conditions inside the triboelectrical measuring chambers.
The measuring probes"" design include separately arranged shells or other probes with definite flow resistance values which will be further discussed herein below.
Also, horizontal or vertical flow conditions at probes situated in central locations or other places are possible, after having increased the dust and aerosol particles"" velocity in advance.
The triboelectric signals are conducted by appropriate electric cables and transmitted to further processing.
The probe for sampling the dust-laden gas and/or dust-and-aerosol particle mixture can be heated and is designed in such a way that right after the partial gas stream (at a temperature similar to that of the gas main stream) enters the sampling probe a diluent air stream at a definite temperature can be added to the partial gas stream by using a proportioning nozzle. The partial gas stream, after possible dilution now being the measuring gas stream, is accelerated by means of an injector and guided through the triboelectrical measuring chamber.
Using for example a multi-way spherical valve, both triboelectrical measuring chamber and sampling probe can be flushed with flushing air.
A volume flow metering and/or monitoring device is integrated in the gas metering section so that constant conditions can be maintained for the partial gas stream, or computerized compensation is enabled as regards the impact the volume flow fluctuations may have on the triboelectric signal, respectively.